Regional variations in the number and morphology of B chromosomes in Drosophila pseudoananassae Bock, 1971

Abstract

The karyotype excluding sex chromosomes of two subspecies of Drosophila pseudoananassae was originally described as three metacentric pairs in the New Guinea strain (D. p. pseudoananassae) and four metacentric pairs in the Borneo strain (D. p. nigrens), while later studies showed that the fifth chromosome of D. p. nigrens from India and Sri Lanka was smaller than that of D. p. nigrens from Borneo. Here, the D. p. nigrens population was divided into two groups: Group A (Thailand and Myanmar) and Group B (India and Sri Lanka). The number of fifth chromosomes varied from 0 to 5 among individuals within the D. p. nigrens populations, and they were considered to be B chromosomes. The two differently sized fifth chromosomes were shown to pair at the centromere region. The small B chromosome was also found in a strain of D. p. pseudoananassae. Although almost all F₁ females and males between strains of the three population groups of D. pseudoananassae were fully fertile, F₁ males between females of the D. p. nigrens B strain and males of D. p. nigrens A and those between females of the D. p. nigrens B strain and D. p. pseudoananassae were sterile. However, no significant morphological differences were present between these two geographic populations of D. p. nigrens.